Sewage sludge recycling with a pipe cross-reactor

ABSTRACT

An improved process for enhancing the plant nutrient value of relatively low analysis organic waste material (e.g. sewage sludge) involves treating the waste material with an acid and base in a pipe-cross reactor. The process more particularly involves mixing the waste material with water to form a slurry (or initially taking the waste material as a slurry); pumping the slurry to a pipe-cross reactor for reaction with a base, acid, and water to form a melt; spraying the melt onto a recycling bed of fines in a granulator, and flashing off the water contained in the melt as steam; rolling the melt onto recycled fine particles in a granulator to form granulated particles; and drying these granulated particles to form an enhanced plant nutrient value composition (e.g. a fertilizer or soil conditioner having a greater &#34;NPK&#34; value than the original relatively low analysis organic waste material). The invention also includes fertilizer produced according to the improved process, which fertilizers are of the same size and shape and density of commonly used fertilizers. The method advantageously utilizes the heat generated by the exothermic acid-base reaction in the pipe-cross reactor to remove the approximately 80% water from sludge, thus saving large amounts of energy normally used in conventional drying or burning methods, while, at the same time, conserving the intrinsic values of the nutrients and humates contained in the sludge. In one embodiment, the process includes a method of disposing of spent acid from a hot dip galvanizing process or a steel pickling process involving incorporating the spent acid to maintain the low pH of a venturi scrubber used in the improved process.

TECHNICAL FIELD

This invention relates generally to a method of treating organicmaterial to create a fertilizer. More specifically, the inventionrelates to the treatment of organic material, such as sewage sludge,with an acid and base in a pipe-cross reactor.

BACKGROUND

The disposal of sewage sludge is a problem. Current methods of disposingof sewage sludge include incineration, direct land or ocean application,heating and drying the sludge for sterilization and then applying it tothe land, depositing it in a landfill, or granulating it with a standardrotary granulator, with heating and drying being provided by exogenousheat sources (e.g. by burning purchased fuel). While some of thesemethods result in what is termed a "fertilizer", such fertilizer is ofrelatively low analysis with regard to its "plant nutrient value".

Methods of expressing a fertilizer's "plant nutrient value" involveidentifying the fertilizer's "NPK" value, wherein N relates to theamount of nitrogen, P relates to the amount of phosphorus (expressed asP₂ O₅), and K relates to the amount of potassium (expressed as K₂ O).Thus, as reported by Wilson in U.S. Pat. No. 3,050,383 (Aug. 21, 1962),sewage sludge with a 2.5-2.5-0 value contains two and a half percentnitrogen, two and a half percent phosphorous as P₂ O₅, and zero percentpotassium as K₂ O. Except as otherwise indicated by usage, allpercentage values used herein are weight-based percentages (i.e. w/w).

Fortunately, methods exist for enhancing the nutrient value ofrelatively low analysis organic waste material. For instance, in theaforementioned Wilson patent (the contents of the entirety of which areincorporated by this reference), a method is disclosed for treatingdried animal manure and sewage sludge with controlled amounts of anacid, such as sulfuric acid, phosphoric acid (or an equivalentphosphorous compound, the strength of which is expressed as phosphoricacid), or mixtures thereof, and an aqueous ammoniacal solution, such asaqueous ammonia or ammoniacal nitrogen salt-containing solutions andtumbling the resulting reaction mass to form fertilizer granules havingan "upgraded" or "enhanced" plant nutrient value.

Other methods of enhancing the plant nutrient value of relatively lowanalysis organic waste material with acids, bases, or mixtures thereofhave also been described. See, e.g., U.S. Pat. No. 4,743,287 (May 10,1988) to Robinson, U.S. Defensive Publication T955,002 (Feb. 1, 1977) toNorton et al., U.S. Pat. No. 5,466,273 (Nov. 14, 1995) to Connell, U.S.Pat. No. 5,125,951 (Jun. 30, 1992) to Lahoda et al., U.S. Pat. No.5,118,337 (Jun. 2, 1992), U.S. Pat. No. 5,393,317 (Feb. 28, 1995) toRobinson, and U.S. Pat. No. 5,422,015 (Jun. 6, 1995) to Angell et al.

A further drawback of sludges treated in conventional manners (e.g. bydrying and screening) is that they are usually of insufficient size andshape to be spread by commonly used agricultural fertilizer spreaders,and cannot be used in the newer pneumatic spreaders.

It would be an improvement in the art if a relatively simple processexisted for processing relatively low analysis organic waste material toan enhanced plant nutrient value composition, especially if such aprocess yielded a product which was capable of being spread by presentlycommercially available spreaders.

DISCLOSURE OF THE INVENTION

The invention includes an improved process for enhancing the plantnutrient value of relatively low analysis organic waste material, suchas sewage sludge. The improvement involves exothermically treating therelatively low analysis organic waste material with an acid and a basein a pipe-cross reactor.

More particularly, the improved process involves mixing the relativelylow analysis organic waste material with water to form a slurry (ortaking the waste material as a slurry); pumping the slurry to apipe-cross reactor for reaction with a base, acid, and water to form amelt; spraying the melt onto a recycling bed of fines, and flashing offthe water contained in the melt as steam. The melt is then rolled onto asubstrate such as recycled fine particles in a granulator to formgranulated particles, causing the granulated particles to grow in size(e.g. to form granules). These granulated particles are then dried (e.g.with a rotary dryer) to reduce their moisture content, and forms anenhanced plant nutrient value composition (e.g. a fertilizer or soilconditioner having a greater NPK value than the original relatively lowanalysis organic waste material).

Generally, the process will also include passing the dried granulatedparticles to a separation apparatus and separating the dried granulatedmaterial into fines, product, and oversized material, and furtherincludes grinding the oversized material and returning the fines andoversized material to the granulator for use as a granular substrate.Potash and other micronutrient materials may be added as dry material tothe returning fines for further enhancement of the product.

The invention also includes fertilizer produced according to theimproved process. Fertilizers produced by the instant invention are ofthe same size and shape and density of commonly used fertilizers.

An advantage of the method is that it uses the heat generated by theexothermic acid-base reaction in the pipe-cross reactor to remove theapproximately 80% water from sludge, thus saving large amounts of energynormally used in conventional drying or burning methods, while, at thesame time, conserving the intrinsic values of the nutrients and humatescontained in the sludge. The method also handles the processed materialas a slurry, thus avoiding the nuisance of conveying and handling dry orsolid materials. The method also achieves high temperatures which aidsin the destruction of pathogens.

In one embodiment, the process includes a method of disposing of spentacid from a hot dip galvanizing process or a steel pickling processcomprising incorporating the spent acid to maintain the low pH of aventuri scrubber used in the improved process thus producing amicronutrient enriched fertilizer.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a process flow diagram, showing a process according to theinvention.

FIG. 2 is a stylized view of a pipe-cross reactor for use with theinvention.

FIG. 3 is a partially cut away, perspective view of a pipe-cross reactorin a rotary ammoniator-granulator for use in practicing the invention.

FIG. 4 is a stylized end view of a rotating bed of materials in agranulator for use with the invention.

BEST MODE OF THE INVENTION

As depicted in FIG. 1, an improved process for enhancing the plantnutrient value of organic waste material generally involves mixing theorganic waste material with water in an agitation tank or sludge slurrytank 10 to form a slurry. The water used in making the slurry preferablyincludes scrubber water from the hereinafter described scrubbers 22which include waste acid in the scrubber water. The slurry is mixed at asufficient concentration and consistency that it will optimally processthe organic waste material as quickly as possible, but will not clog orblock the pipe-cross reactor 12 during operation. Of course, theparticular slurry concentrations and consistencies will depend, to someextent, on the size and amount of insoluble particulate materialcontained in the particular organic waste material, and the size andlength of the pipe-cross reactor components.

The slurry is pumped from the agitation tank 10 to a pipe-cross reactor12 for an exothermic reaction with, for example, a base such as ammonia,and an acid or acids such as sulfuric acid, phosphoric acid, andmixtures thereof, with or without extra water to form a melt.

Pipe-cross reactors are well-known, and have been used in the past toproduce granular NPKS fertilizers from liquid chemicals. See, e.g.,Energy Efficient Fertilizer Production with the Pipe-Cross Reactor,(U.S. Dept. of Energy, 1982) (a pipe-cross reactor fit into thegranulator drum of a conventional ammoniation-granulation system);Achorn et al., "Optimizing Use of Energy in the Production of GranularAmmonium Phosphate Fertilizer" (1982 Technical Conference of ISMA,Pallini Beach, Greece); British Sulfur Corp. Ltd., "TVA modifies itspipe reactor for increased versatility", Phosphorus & Potassium, No. 90,pp. 25-30 (1977); Achorn et al., "Efficient Use of Energy in Productionof Granular and Fluid Ammonium Phosphate Fertilizers" (1982Fertilization Association of India Seminar, New Dehli, India); Salladayet al. "Commercialization of the TVA Pipe-cross Reactor in Regional NPKSand DAP Granulation Plants in the United States" (1980 FertilizationAssociation of India seminar, New Dehli, India); U.S. Pat. No. 4,619,684(Oct. 28, 1986) to Salladay et al.; U.S. Pat. No. 4,377,406 (Mar. 22,1983) to Achorn et al.; U.S. Pat. No. 4,134,750 (Jan. 16, 1979) toNorton et al.; U.S. Defensive Publication T969,002 (Apr. 4, 1978) toNorton et al; and Salladay et al. "Status of NPKSAmmoniation-Granulation Plants and TVA Pipe-cross Reactor" (1980Fertilizer Industry Round Table, Atlanta, Ga. US).

Amounts of acid and base used in the exothermic process can bedetermined by one of skill in the art. However, for guidance, in theneutralization of ammonia, approximately one mole of sulfuric acid, orof phosphoric compounds expressed as phosphoric acid, is used for eachtwo moles of ammonia. Concerning the concentration of phosphoric acid,typical molar ratios of N:P in the pipe-cross reactor are between 1.3:1to 1.8:1, preferably 1.4:1 and 1.5:1, taking into consideration waterdilution of the phosphoric acid to between about forty-two toforty-seven (42 to 47%) P₂ O₅. The molar amount of nitrogen should takeinto consideration not only the amount of ammonia being added, but thetypical amount of ammoniacal nitrogen contained in the particularorganic waste material.

Other acids which may be used with the invention include nitric acid,hydrochloric acid, acetic acid, citric acid and mixtures thereof.Certain combinations (e.g. nitric acid and an ammonia compound whichmight form ammonium nitrate which may be explosive) need to be carefullyconsidered before use however. Whatever the acid or acids chosen, thestrength of one of the acids used in the process will preferably beequivalent to 90% sulfuric acid (e.g. 93 to 100 percent sulfuric acid).

As depicted in FIG. 2, the pipe-cross reactor 12 is preferably providedwith two cross pipes 26, 28 to receive sulfuric acid (at a rate of about8.6 gpm) and phosphoric acid (at a rate of from about 2.6 gpm). A thirdpipe 30 incorporates the ammonia into the center of the reactor. Thelength of this pipe 30 should be at least twenty (20) to thirty (30)inches to ensure adequate mixing. A third cross pipe 32 incorporated theslurry and additional water as required into the mixing chamber. Atypical pipe-cross reactor for use with the invention has a diameter ofapproximately three (3) to ten (10) inches, is from about seven (7) toabout fifty (50) feet long, and terminates in a, for example, two (2) toeight (8) inch discharge pipe (or a slot of equivalent cross-sectionalarea), preferably with a stainless steel insert or TEFLON™ lining. Thedischarge pipe preferably discharges into a standard rotating drumgranulator 14. It is preferably made of a steel pipe (e.g. HASTELLOYC-276 or 316L stainless steel (with HASTELLOY C or B for the reactiontube)). A TEFLON™, ceramic, or other corrosion resistant lining may alsobe used in the pipe-cross reactor. The temperature is preferablymaintained below 149° C. (300° F.).

The ammonia is introduced into the system at a rate of from about 4.3gpm. Organic waste material (e.g. sewage sludge) and water areincorporated at a rate of from about 30 to about 40 gpm of slurry. Thepipe-cross reactor is typically operated at a gage pressure of betweenfifteen (15) and sixty (60) psig.

A "hot melt" discharges from the pipe-cross reactor tube into thegranulator 14, while water flashes from the reactor product as it issuesinto the granulator 14. Steam is generated by the exothermic reactionconducted within the pipe-cross reactor 12.

A preferred granulator (also commonly known as an"ammoniator-granulator") is a two (2) to four (4) meter (e.g. six (6) totwelve (12) feet) diameter rotating drum granulator having a length offrom about five (5) to about seven (7) meters. In the depicted process,the granulator 14 includes an ammonia sparger 20 operably positionedwithin the granulator 14 for the addition of small amounts of ammonia tothe melt to, for example, control or adjust the pH of the granulatedmaterial.

The melt is rolled onto recycled fine particles within the granulator 14to form granulated particles, thus causing the granulated particles togrow to a desired size. Afterwards, these granulated particles arepassed into a rotary dryer 16 for a sufficient amount of time to reducetheir moisture content, and forming a fertilizer having an enhancedplant nutrient value.

A preferred dryer for use with the invention is a two (2) to three (3)meter (e.g. six (6) to eight (8) feet) diameter rotating drum dryerhaving a length of from about fifteen (15) to about seventeen (17)meters, and having a heating capacity of 30 to 45 million BTU/hour, witha lump crusher at the discharge end.

The depicted process further includes passing the dried granulatedparticles to a separation apparatus 18 and separating the driedgranulated material into fines, product and oversized material.Oversized material is reduced in size to be incorporated, as a fine,into the process.

The fines are returned to the granulator 14 (along with potash or anymicronutrients required for the desired final product analysis) forincorporation into the process. The product from the separation processis preferably cooled in a product cooler (from two (2) to three (3)meters in diameter, and fifteen to seventeen (17) meters long) or asuitable fluid-bed type cooler.

During the process, fumes from the granulator 14 containing steam,ammonia, and particulate are collected by maintaining a negativepressure inside the granulator 14 by pulling the fumes through a venturiscrubber 22 having low pH water as scrubber water sprayed into theventuri throat.

Other aspects of a ventilation system for use with the inventionpreferably include fans (e.g. ones capable of moving about 60,000 cubicfeet per minute of air), dry cyclones for dust collection, and venturiscrubbers with water separation chambers for collecting ammonia fumesand small dust particles.

The invention uses low pH water in the venturi scrubbers to collectunreacted ammonia vapors escaping the granulator. In one embodiment,small amounts of sulfuric or phosphoric acid are added to the venturiscrubbers to maintain a low pH (e.g. 2 to 3) for proper ammonia vaporscrubbing in the venturi scrubbers.

NPK fertilizers generally however preferably include the micronutrientsiron and zinc. In a preferred embodiment therefore, spent acid from ahot dip galvanizing (EPA/RCRA hazardous waste designation D002 D006,D007 and D008) or steel pickling process (EPA/RCRA hazardous wastedesignation K062) is used to maintain the low pH of the scrubber water.These spent acids commonly are sulfuric acid or hydrochloric acid offive (5%) to ten percent (10%) strength, containing three (3) to eight(8) percent iron. Galvanizing spent acid contains three (3) to eight (8)percent zinc along with the previously described iron. The iron and zincare fed with the ammonia-laden scrubber water from scrubbing to thesludge slurry tank, and on to the pipe-cross reactor for incorporationas iron and zinc micronutrients in the final NPK fertilizer. In the caseof spent sulfuric acid, the sulfur also becomes a nutrient in theresulting fertilizer, since it reacts in the pipe-cross reactor to formammonium sulfate (while hydrochloric acid goes to form ammoniumchloride).

Other micronutrients or additional ingredients may be incorporated intothe resulting fertilizer by adding them with a weigh feeder as a drysolid to the fines recycle stream. "Micronutrients" or "additionalingredients" include lime, dolomite, calcite, hydrobiotie, gypsum,phosphates (e.g. rock phosphate or ammonium phosphate), potash, urea,soil clays, calcium peroxide, ammonium nitrate, vermiculite, humic acid,and trace minerals such as iron, manganese, magnesium, boron, copper,and zinc.

Although the invention has been most particularly described for theprocessing of municipal sewage sludge, the inventive process may also beused to enhance the plant nutrient value of other relatively lowanalysis organic waste material such as poultry manure, food processingwastes, wastes from paper manufacturing, swine manure sludge, mixturesthereof, and the like. In such a case, the particular relatively lowanalysis organic waste material is substituted for the sewage sludge inthe process, and the process parameters are accordingly modified.

The invention is further explained by the following illustrativeexample:

EXAMPLE

In an agitation tank, 6700 kilograms/hour (7.4 tons/hour) of sewagesludge are mixed with 37 liters per minute (ten gallons/minute (gpm)) ofscrubber water from a venturi scrubber to form a slurry. The slurry isof a consistency that it can be pumped with a positive displacement pumpor other suitable pump to a pipe-cross reactor equipped to receiveammonia, sulfuric acid, phosphoric acid, sewage sludge, and water. Thepipe-cross reactor has a diameter of approximately four (4) inches, andis forty (40) feet long. The pipe-cross reactor terminates in a rotatingdrum granulator. The rotating drum granulator is six (6) feet indiameter, and is twenty (20) feet long.

The slurry is added to the pipe-cross reactor, and is reacted with 8.6gpm 99.5% ammonia, 8.6 gpm sulfuric acid (93%), and 2.6 gpm phosphoricacid (54% P₂ O₅). The temperature of the pipe-cross reactor (due to theexothermic reaction between the acid and the base) is maintained atabout 149° C. (300° F.) with moisture in the sludge. This temperatureacts to kill Salmionella, E. coli, and other pathogens which may befound in the slurry. This temperature also acts to de-odorize thematerial somewhat.

The resulting melt from the pipe-cross reactor is sprayed onto arecycling bed of fines along with 2000 pounds of added potassiumchloride (60% K₂ O) while the water contained in the melt flashes off assteam. An ammonia sparger is provided in the granulator to add smallamounts of ammonia to the granulation mixture for pH control.

Operating the pipe-cross reactor in such a manner incorporatesapproximately twenty (14.8) tons per hour of 20% solids sewage sludge ata ten (10) ton per hour production rate.

Fumes from the granulator containing steam, ammonia and particulate arecollected by maintaining a negative pressure inside the granulator witha fume fan pulling fumes through a venturi scrubber with low pH water(water at a pH lowered by the addition of spent acid from a hot dipgalvanizing process) sprayed into the venturi throat. (If galvanizingacid is unavailable, the pH may be controlled with phosphoric orsulfuric acid). The low pH water collects ammonia vapor present in thefumes, as well as dust particles.

Granulated material exits the granulator at about 93° C. (200° F.) andat about a five (5) to fifteen (15) percent moisture content into arotary dryer. The rotary dryer is approximately two meters (e.g. six (6)feet) in diameter and has a length of about twenty meters (e.g. sixty(60) feet). It has a heating capacity of 30 million BTU/hour, and isassociated with a lump crusher or lump breaker at the discharge end. Themoisture in the material is reduced to about three percent (3%) byheated forced air in the dryer.

Materials exiting the rotary dryer are run through the lump crusher toreduce oversized material to less than one (1) inch in size.

Screens are used to separate the material into (a) fines, (b) productand (c) oversized material. Fines are returned to the granulator.Product goes to a two meter (six foot) diameter, twenty meter (sixtyfoot) long, cooler and then on to storage, while the oversized materialis passed through a grinding mill, and reduced to fines for recycling tothe granulator. About two (2) tons (1800 kg) of fine material per ton ofproduct are required in the recycle stream.

Dust-laden air is collected from the dryer, grinding mills, and screensby a fan maintaining negative pressure on all of the equipment. The airis pulled through a cyclone system that removes about 97% of the dust.From the cyclones, the air is passed through a venturi scrubber toremove the remaining dust particles. Air from the venturi scrubber issent to a large separator chamber, along with the air from thegranulator fume scrubber to remove any condensed moisture. The air fromthese venturi scrubbers is combined and passed through a secondaryventuri scrubber. The air then exits through a stack approximately onehundred (100) feet tall. The air is saturated at around 66° C. (150°F.).

The resulting fertilizer is determined to have an NPK value of 12-3-6(12% nitrogen, 3% phosphate, and 6% potash). It is also homogenous andproperly sized for standard application equipment.

References herein to a specific Example or specific embodiments shouldnot be interpreted as limitations to the invention's scope which isdetermined by the claims.

What is claimed is:
 1. A process of treating relatively low analysisorganic waste material to form an enhanced plant nutrient valuecomposition, said process comprising:mixing said relatively low analysisorganic waste material with water to form a slurry capable of beingpumped; pumping said slurry to a pipe-cross reactor for reaction with abase, acid, and water to form a melt; spraying said melt onto arecycling bed of fines in a granulator, and flashing off water containedin the melt as steam; rolling said melt onto fine particles in thegranulator to form granulated particles; and drying said granulatedparticles to reduce the moisture content thereof to form driedgranulated particles comprising an enhanced plant nutrient valuecomposition.
 2. The process of claim 1 further comprising:passing thedried granulated particles to a separation apparatus and separating saiddried granulated material separate the material into fines, product andoversized material.
 3. The process of claim 2 furthercomprising:returning the fines along with added potash and othermicronutrients to the granulator as fine particles to form a substratefor the granulated particles.
 4. The process of claim 1 wherein thegranulator includes an ammonia sparger for adding ammonia to the melt.5. The process of claim 1 wherein fumes from the granulator containingsteam, ammonia and particulate are collected by maintaining a negativepressure inside the granulator by pulling the fumes through a venturiscrubber having a venturi throat with low pH water as scrubber watersprayed into the venturi throat.
 6. The process of claim 5 wherein thepH of the scrubber water is kept low by incorporating spent acid from ahot dip galvanizing process into the scrubber water.
 7. The process ofclaim 5 wherein the pH of the scrubber water is kept low byincorporating spent acid from a steel pickling process into the scrubberwater.
 8. The process of claim 1 wherein a rotary drier is used to drythe granulated particles.
 9. The process of claim 8 wherein the rotarydryer has a lump crusher associated therewith.
 10. The process of claim9 wherein materials exiting the rotary dryer are run through the lumpcrusher to reduce oversized material to less than one (1) inch in size.11. The process of claim 1 wherein the relatively low analysis organicwaste material is selected from the group consisting of sewage sludge,poultry manure, food processing wastes, wastes from paper manufacturing,swine manure sludge, and mixtures thereof.
 12. A process of disposing ofspent acid from a hot dip galvanizing process or a steel picklingprocess comprising incorporating said spent acid to maintain the low pHof a venturi scrubber used in a process according to claim 5.